1. Field of the Invention
Embodiments of the present invention generally relate to thermal processing of thin films on substrates such as a silicon wafers. In particular, embodiments of the invention relate to methods and apparatus used in detecting lamp failure for an array of lamps used to produce radiation for such thermal processing.
2. Description of the Related Art
Rapid thermal processing (RTP) is one thermal processing technique that allows rapid heating and cooling of a substrate such as a silicon wafer. Typical peak processing temperatures can range from about 450° C. to about 1100° C. and can be applied for about 15 to about 120 seconds before wafer cool down begins. The specific peak temperature and heating time used depend on the type of wafer processing. RTP wafer processing applications include annealing, dopant activation, rapid thermal oxidation, and silicidation among others. The rapid heating to relatively high temperatures followed by the rapid cooling that characterize RTP provides more precise wafer processing control. For example, RTP annealing following ion implantation of dopants allows repair of crystal damage while minimizing the diffusion of the dopant atoms due to the very short heating time. The crystal damage can be repaired before the implanted atoms can move from their original location. Other thermal processing techniques with longer heating and cooling cycles cannot achieve comparable dopant diffusion control during annealing.
The trend for thinner oxides used in MOS gates has led to requirements of oxide thicknesses less than 100 Angstroms for some device applications. Such thin oxides require very rapid heating and cooling of the wafer surface in an oxygen atmosphere to grow such a thin oxide layer. RTP systems can provide this level of control, and are used for rapid thermal oxidation processing. The technique of RTP uses the principle of radiation heating to allow rapid heating and cooling. Typically, this radiation is provided by many lamps placed in an array that is located above the wafer surface. The radiation from the many lamps heats the wafer surface and brings it up to process temperature in a matter of seconds. Since the lamps are electrically powered, they can be turned on and off quickly. The short heating time allows heating of the wafer surface without substantially heating the RTP chamber. This allows rapid cooling of the wafer surface when power to the lamps is turned off. The rapid heating and cooling cycle also reduces the thermal budget needed for the process. The reduced cycle time can also be used to decrease total processing time and increase wafer throughput. A result of the short heating cycle used in RTP is that any temperature gradients that may exist across the wafer surface can adversely affect wafer processing. It is, therefore, important in RTP to monitor the temperature across the wafer surface and ensure temperature uniformity in and on the wafer surface during processing. As a result, lamp placement and the control and monitoring of individual lamps are important so that the radiation output can be controlled to help ensure temperature uniformity across the wafer surface.
FIG. 1 shows a partially sectioned orthographic view of an RTP system 10. A silicon carbide wafer support ring 24 is supported on a rotating quartz cylinder 22. The wafer support ring has a pocket 32 into which a wafer (not shown) can be placed. A lamphead 14 faces the wafer support ring. The lamphead includes several hundred tungsten halogen lamps 26 that form an array of lamps which faces the wafer. A typical rating for such a lamp is in the range of 500 W to 650 W, and the tungsten halogen lamps emit strongly in the infrared. The bulb portion 42 of lamp 26 is shown in FIG. 3. The tubular bulb is typically made of quartz, filled with a halogen containing gas, and then sealed around two outer filament leads 50 and 52. A nose tip 46 remains after sealing. Enclosed within the sealed bulb is a tungsten filament 44 that is helically wound, with one end connected to a filament lead 52 and the other end connected to a side arm support 48. The most common mode of lamp failure is shorting of a few turns of the helical filament. Referring back to FIG. 1, each lamp is held in a stainless steel sleeve 16 potted into a water cooled stainless steel housing 18. The lamp bulbs extend beyond the sleeves 16 and the housing 18 into a front plate 30 which has an array of thru holes which match the lamp array. A reflector 20 is inserted into each thru hole. A thin quartz window 28 is located between the open end of the reflectors 20 and the chamber space 12 above the wafer.
FIG. 2 is another view of the front plate 30 which more clearly shows how the lamps may be arrayed. In this example, the lamps 26 are in a hexagonal array. The center lamp 26A is located on the wafer rotation axis 34. The wafer is rotated so that it will see a more uniform radiation distribution. The lamp array pattern plus wafer rotation is one approach to creating a more uniform distribution of radiation and temperatures across the wafer surface. However, this approach alone will not usually produce the temperature uniformity needed, and so typically the lamps may be controlled in concentrically arranged zones, for example, fifteen zones, such that the lamp power can be adjusted for each zone to compensate for thermal effects at the wafer center and edge to produce a more uniform radial temperature profile.
Variation in lamp intensity due to lamp failure or poor performance can greatly compromise the desired temperature profile control and result in unacceptable process results. Accordingly, a monitoring system that can detect lamp failure or unacceptable lamp performance prior to wafer processing is a useful feature for an RTP system. FIG. 4 is a schematic representation of a prior art lamp failure detection system for an RTP system. The lamps are powered by a silicon controlled rectifier (SCR) driver 60. The lamphead contains several hundred tungsten halogen lamps which are divided into multiple, radially symmetric zones, and each zone is separately powered by an SCR driver so that the lamp power can be adjusted for each zone. Each zone contains multiple lamps, and the lamps are divided into pairs with each lamp pair connected to the SCR driver. The two lamps of each pair are connected in series. In the present example, such a lamp pair is represented by lamps L1 and L2, which are included in a power distribution board 64. The power distribution board contains all lamps in the lamphead, but only a single lamp pair is shown since the same lamp failure detection circuit is applied to each lamp pair. The power distribution board which includes lamps L1 and L2 is connected to a lamp failure detection (LFD) board 62. The LFD board includes a current transformer sensor 66 which is magnetically coupled to a conducting line 68 so that the current passing through lamps L1 and L2 can be measured. The conducting line 68 may be a printed circuit board trace. The sensor is connected to a comparator 74 which can compare the current measured to a pre-set threshold value to determine if a failure condition exists. In this example, a failure condition would be detected if the measured current were less than the threshold current value. This information is then sent to an operator display screen which identifies the particular lamp pair that is in a failure state. For example, if the lamp L2 filament were to break, this open filament condition would create an open circuit and result in zero current flow through lamps L1 and L2. The current sensor would then detect a lamp failure state.
The lamp failure detection system shown in FIG. 4 has several limitations. If one of the lamp filaments breaks, the system cannot detect which lamp L1 or L2 has the open filament since the failure detection method measures current for two lamps connected in series. As a result, it is necessary to check both lamps for failure if a failure state is indicated for the lamp pair. Also, the lamps for a given pair are often located at some distance apart within the lamphead to minimize the impact to radiation uniformity should one of the lamps fail during wafer processing. Significant time could be saved if only the failed lamp had to be located within the lamp array, resulting in decreased down time for the RTP system.
Another limitation of the prior system is that it cannot detect different types of lamp failure. The use of current measurement to detect lamp failure for two lamps in series has inherent limitations since the measured current value is a result of the combined resistance of both lamps. If one of the lamp filaments is open, then the absence of current will trigger a failure signal since the current is now below the threshold value. It is also possible that a lamp may have a partial short which would decrease the lamp resistance and increase the current measured by the sensor. This would not trigger a failure signal since the current would remain above the threshold value. A lamp with a partial short will tend to have a radiation output that differs from the output of a normal lamp. The change in radiation output could adversely effect wafer processing. In the case of an incandescent light source such as a tungsten halogen lamp, a partial short can occur from the shorting of a few turns of the helical filament, which will typically alter the lamp radiation output and shorten the lamp lifetime.
An additional limitation of the prior system is suggested by the current waveforms shown for sensor input 70 and sensor output 72 for normal lamp operating conditions. The current transformer 66 has a minimum threshold value for current rate-of-change. If the input signal waveform has a rate-of-change that is below this threshold value, the current sensor will not function. This implies that the voltage and current waveforms must meet certain requirements in order to use the current transformer 66 for detecting current. The input waveform 70 does meet such requirements; a low frequency sine-wave, for example, may not. Also, because the current sensor 66 is magnetically coupled to conducting line 68, the sensor is susceptible to any noise created by stray electromagnetic fields near the RTP system. This noise can degrade the accuracy of current measurement, and hence the accuracy of the lamp failure detection system.
Therefore, there is a need for an improved apparatus and method for lamp failure detection. It would be useful to have a lamp failure detection system that is independent of voltage and current waveforms, and can function accurately and reliably in the presence of stray electromagnetic fields. Also, it would be useful to have a failure detection system that can identify which lamp has failed, and identify the type of failure, such as a partial short. More generally, it would be useful to have a failure detection system which can detect any deviation from the normal operating characteristics of a lamp. Such information can be used to reduce system downtime as well as help prevent lamp failure during wafer processing.